1. Field of the Invention
The present invention relates to a digital-to-analog converter, and more particularly, to a digital-to-analog converter based on a pre-decoder, a binary decoder and ROM decoders.
2. Description of the Prior Art
Because liquid crystal display (LCD) devices are characterized by thin appearance, low power consumption, and low radiation, LCD devices have been widely applied in various electronic products such as computer monitors, mobile phones, personal digital assistants (PDAs), or flat panel televisions. The operation principle of an LCD device is to vary voltages dropped on two terminals of liquid crystal cells in order to change the twisted angle of the liquid crystal molecules so that the transparency of the liquid crystal cells can be changed for illustrating images. As to most of portable communication devices using the LCD devices and having limited power provided by the battery devices, how to provide LCD devices with higher power efficiency, lower production cost, and smaller size to meet customer value has become a key issue to the future display development.
Please refer to FIG. 1, which is a block diagram schematically showing a conventional LCD device 100. The LCD device 100 comprises a source driving device 110 and a display panel 190. The source driving device 110 comprises a latch control shift unit 120 and a plurality of source driving circuits 130_1-130_n. The latch control shift unit 120 comprises a plurality of shift registers 120_1-120_n. The plurality of shift registers 120_1-120_n are used for inputting pixel data signals into the plurality of source driving circuits 130_1-130_n. Each of the plurality of source driving circuits 130_1-130_n comprises a latch 140, a digital-to-analog converter (DAC) 150, and a buffer 160.
The display panel 190 is coupled to the plurality of source driving circuits 130_1-130_n via a plurality of data lines DL_1-DL_n. The pixel data signal corresponding to each of the data lines DL_1-DL_n is inputted to the corresponding latch 140 and is converted into an analog pixel data signal by the corresponding digital-to-analog converter 150. The analog pixel data signal is outputted through the corresponding buffer 160 to the display panel 190 via one corresponding data line.
It is obvious from the above description that each of the plurality of source driving circuits 130_1-130_n comprises one digital-to-analog converter 150 for performing a digital-to-analog converting process on the corresponding pixel data signal in digital form. That is, if there are 1024 data lines for inputting analog pixel data signals to the display panel 190, then the source driving device 110 requires 1024 digital-to-analog converters 150. In this way, if each of the plurality of digital-to-analog converters 150 consumes a significant device area, then a large circuit layout area for the source driving device 110 of the LCD device 100 is required and hence it is hard to achieve thinner appearance and to cut down the related cost.
Please refer to FIG. 2, which is a circuit diagram schematically showing a prior-art digital-to-analog converter 290 based on ROM decoder architecture. The digital-to-analog converter 290 comprises a plurality of transistor arrays 200-263 and a buffer 280. The plurality of transistor arrays 200-263 are utilized to select one of gamma reference voltages Vr0-Vr63 based on an 6-bit digital data signal D0-D5 and a complementary 6-bit digital data signal D0B-D5B so as to output an output voltage Vout via the buffer 280. Each of the plurality of transistor arrays 200-263 comprises 6 transistors, and hence the plurality of transistor arrays 200-263 comprises 384 transistors. Accordingly, the prior-art digital-to-analog converter 290 consumes a significant device area, and therefore a large circuit layout area for the source driving device of an LCD device using a plurality of the prior-art digital-to-analog converters 290 is required.